A bias circuit is connected to an amplifier and applies a constant bias voltage with respect to an input signal to the amplifier.
A circuit 100 illustrated in FIG. 7 includes a constant-current source (CS1) 101, first and second transistors (MN1, MN2) 102, 104, and resistor 103. Of these components, a bias circuit is configured by the constant-current source (CS1) 101 and the first transistor (MN1) and applies a bias voltage to an amplifier configured by a coil 106 and a transistor (MN3) 105.
A variety of bias circuits have heretofore been suggested (see, for example, the following Patent Document 1 to Patent Document 3).    Patent Document 1: Japanese Laid-Open Patent Publication No. H9-148853.    Patent Document 2: Japanese Laid-Open Patent Publication No. H11-234060.    Patent Document 3: International Publication Pamphlet No. WO 01/08299 A1.
In the circuit 100 illustrated in FIG. 7, the gate length of the first transistor (MN1) 102 may be made less than the gate length of the second transistor (MN2) 104 in order to apply Vth (threshold voltage) to the gate of the second transistor (MN2) 104. With such a configuration, a characteristics of the amplifier is restricted because the transistor (MN2) 104 with a large gate length is used regardless of the possibility of producing the transistor (MN1) 102 with a small gate length in the transistor production process. Further, where such the circuit 100 is produced, because the first and second transistors (MN1, MN2) 102, 104 have different gate lengths, the effect of production spread is easily demonstrated, the bias accuracy is degraded, and the yield is reduced.
In all the bias circuits described in the aforementioned patent documents, Vth+α is taken as a bias voltage with respect to the threshold voltage Vth. However, although Vth+α is applied as the bias voltage, a B-class amplifier does not operate as the B-class amplifier. This is because it starts operating as the B-class amplifier after the threshold voltage Vth has been applied. It is desirable to configure a bias circuit that will apply the threshold voltage Vth to an amplifier with nonlinear input and output, such as the B-class amplifier.